In heat exchangers built up by plates with mutual intersecting corrugations it is known to change the flow resistence of the heat exchange passages and consequently also the so called thermal length by varying the press depth and the mutual angle of the corrugations of adjacent plates and by combining different press depths and angles. The possibilities to influence the flow characteristics of the passages with such arrangements, however, are limited to changes equal in size of the passages for the two media. A change of the passages for one of the media thus causes a corresponding change of the passages for the other medium.
The above mentioned limitation is a drawback since it is sometimes desirable to be able to bring about asymmetrical passages, i.e. to change the flow characteristics of the passages for the two media independently of each other, for instance when having the same type of medium in liquid state and the same allowed pressure drop and essentially the same viscosity and when the flows of the media are unequal in size, i.e. when the task of the heat exchange is asymmetrical. The heat exchanger in this example must be dimensioned for that medium that has the largest flow such that desired pressure drop is achieved in the passages through which this medium passes. Due to this fact the passages for the other medium, which have the same capacity, will be over-dimensioned for the actual flow. Which medium that becomes limiting depends on size of flow, state of aggregation, the highest allowed pressure drop, type of fluid etc. Thus, also during condensation and/or evaporation the passages for one of the media usually becomes limiting, while the upper limit of the pressure drop for the other medium cannot be utilized. Accordingly, the heat exchanging surfaces of the apparatus are not utilized in the best way, which is unfavourable from an economic point of view.
In order to master this problem heat exchanger plates have been suggested provided with an unsymmetrical corrugation pattern having narrow ridges and wide grooves or vice versa. By means of such plates it is possible to bring about a heat exchanger, in which the passages for the two media have mutual different volume and consequently different flow characteristics. The difference in flow characteristics achieved in this way, however, is small at the same time as the area enlargement of the pattern has to be reduced. Therefore, this solution has appeared not to be so suitable in practice.